The ink jet printing technology is well known in the sector art and has been described in numerous patents and publications.
In short, ink jet printing is based on the ejection of droplets of ink from a head; the different nature of the droplet ejection modes distinguishes thermal ink jet printers from piezoelectric type ones.
Thermal ink jet printing technology is identified by the acronym TIJ—Thermal Ink Jet—and it is to this type that this invention relates.
Depicted in FIG. 1 is a section of an ejector assembly 55 of a thermal type ink jet head in which the following are indicated:                a tank 103 (generally a sponge), connected through a slot 102 to the ejector assembly 55, which contains ink 142;        an inlet channel 53;        a chamber 57 located in correspondence with a resistor 27;        a nozzle 56;        arrows 52, indicating the prevalent direction of motion of the ink 142 from the tank 103 to the nozzle 56;        a nozzle plate 106, generally though not exclusively made of a lamina of gold-plated nickel or kapton;        a vapour bubble 65;        a meniscus 54 local to the surface of separation, generally referred to as interface by those skilled in the art, between the ink 142 and the air; and        a droplet 51 of ink.        
The energy required for ejecting the ink droplet 51 is supplied by the resistor 27, which heats rapidly through the effect of a current resultant on a signal coming from a microprocessor. The heating creates the vapour bubble 65 which causes the expulsion of the ink droplet 51 through the nozzle 56.
During the ejection process, in correspondence with each nozzle 56, the meniscus 54, by breaking and forming again, regulates the correct formation and expulsion of the ink droplet 51, as is known to those skilled in the art.
The printed images are formed by the ink dots that the ejected droplets form upon the physical medium, for example the paper. The quality of the images obtained with ink jet printing depends on a variety of factors, such as for instance the properties of the ink and of the substrate, which include the “spreading” and the penetration of the ink, the roughness of the paper, the water-fastness of the ink once printed, the mechanical design of the head, which includes the disposition of the ejection holes and the paper transport system, the droplet formation, which includes the ejection frequency, the speed and angle of impact of the droplet on the paper, and the printer specifications, which include, for example, resolution, presence or otherwise of “halftoning” and of “dot-on-dot” management, as is known to those skilled in the art.
The most commonly used inks in ink jet printing are:                water-based inks;        inks based on pigment dispersions; and        solvent based inks.        
The latter often present toxicity problems which require a controlled recovery and disposal of the solvent. In the case of pigment-based inks, which give highly vivid colours, the size of the particles of the pigment is often a critical factor as it is accompanied by “clogging” (complete or partial obstruction of the nozzles) in micro-hydraulic systems as required by recent technology.
The majority of inks for ink jet printing use water as the solvent: this invention relates to inks of this type.
In TIJ printing technology, specially formulated inks are used having precise characteristics, among which those in the following non-restrictive list:                ink properties: pH between 6 and 10, viscosity ranging from 1 to 5 mPa*s @ 25° C. and surface tension between 20 and 50 mN/m @ 25° C.;        optical density characteristics conforming to the application of the printer: varyingly vivid colours or with tones depending on the print application and cultural factors, related for instance to the geographical area the product is sold in;        storage stability both from the chemical and microbiological point of view;        complete thermal stability so as not to form insoluble residues on the resistor during the thermal bubble formation process which would compromise subsequent ejections;        ability to produce high quality images on a wide variety of papers and media in general;        perfect operation of the printhead throughout its entire service life;        compatibility with the other materials comprising the printhead; and        safety of use for the user and absence of environmental damage.        
The essential components of the ink are the solvent and the dye, but the following are also found in the various formulations, by way of non-restrictive example:                biocides, which prevent the growth of biological material which could lead to blocking of the holes;        humectants, which reduce the extent of evaporation of the ink during operation of the printer; and        surface-active agents, whose functions are examined in the following.        
As is known to those skilled in the art, a surface-active agent is a substance which, when added in a certain quantity to a liquid system, considerably reduces its surface tension.
The molecules of surface-active agents are amphoteric structures as they contain a hydrophilic part and a hydrophobic part and are characterized by a quantity known to those skilled in the art as HLB (Hydrophylic-Lipophylic Balance).
FIG. 2 is a diagram of a water-based liquid system 70 comprising molecules 74a, 74b and 74c of a surface-active agent, each of which has a hydrophylic part 75 and a hydrophobic part 76. This liquid system 70 is contained in a vessel 20 and is in contact with the air 25 through an interface 61. The mass of the liquid, or “bulk” to those skilled in the art, is designated with the numeral 66.
By its amphotic nature, the surface-active agent is found in the system it is inserted in, such as for example the system 70, in the following ways:                molecules 74a are arranged on the interface 61 with the hydrophobic part 76 facing the outside; this arrangement effectively separates the molecules of the water-based medium which, if the surface-active agent were missing, would be one beside the other at the interface 61, thus diminishing cohesion and thereby lowering surface tension;        molecules 74b form micelles 63 in the bulk of the liquid. These are formed because the molecules of the surface-active agent are above a concentration called “critical micellar concentration”—cmc—leading some molecules which cannot find room at the interface to agglomerate in the bulk in the prevalently spherical shape micelles 63; and        molecules 74c are scattered through the bulk 66.        
A system containing a surface-active agent, such as any liquid system, is characterized by an equilibrium surface tension (τstat), measured under static conditions using the method known as “DuNuoy ring” or the “Wilhelmy plate” method. Under static conditions, the surface-active agent has sufficient time to migrate to the interface 61 and cause the system to reach its final surface tension value.
Under dynamic conditions, when the interface 61 is not steady but is continuously broken and formed again, the surface-active agent tends to restore status of the system to the steady conditions; this tendency is called “rearrangement” by those skilled in the art. A surface-active agent may be capable of migrating quickly to each new interface formed and effectively lowering the surface tension value (in this case, the behaviour under dynamic conditions is known as “fast rearrangement”), or may not be able to rearrange quickly enough to guarantee a migration of all the molecules available for lowering the surface tension, which accordingly remains higher (behaviour under dynamic conditions called “slow rearrangement”).
It is possible to measure surface tension under dynamic conditions, indicated as DST, i.e. Dynamic Surface Tension, by way of a non-restrictive example using a method known as “Maximum Bubble Pressure” which, using a capillary through which air is blown into the liquid under analysis so as to form bubbles, measures surface tension in relation to the speed of formation of the bubbles. This instrumentation makes it possible to investigate the dynamic behaviour of a liquid system containing a surface-active agent at different surface renewal speeds, by means of a computing process known to those skilled in the art.
The surface-active agents present in the formulation of inks determine and stabilize the surface tension value. This parameter is used by head designers since, together with other factors such as the intrinsic viscosity of the ink, it conditions the geometrical sizing of the ejector assembly 55.
Design of the hydraulics of the head comprises the sizing of the inlet channels 53, the chambers 57, the resistors 27 and the nozzles 56 depending on the product objectives established at the outset. The ejection frequency, i.e. the frequency with which the droplets 51 are ejected through the nozzles 56, is limited by the speed at which the chambers 57 are filled by the ink.
The limit frequency beyond which the ejector 55 is no longer able to guarantee a regular ejection and the droplet 51 assumes uncontrolled speed and volume is called “maximum working frequency”.
Filling of the nozzle 56 is regulated, as well as by the resistances to flow given by the ink's viscosity and by the geometrical parameters of the inlet channels, by the capillary pressure P determined according to the following known expression (formula A):P=2*τ*cos(α/r)where:                τ is a generic surface tension of the ink 142;        α is an angle of contact measured between the ink and a surface of the same material as the nozzle plate 106;        r is the radius of the nozzle 56.        
The generic surface tension τ of the ink also determines the interaction of the droplet 51 with the physical medium, generally a sheet of paper. The tendency of the droplet as it impacts on the paper to spread out to varying degrees regulates the type of image the user wishes to obtain.
In fact, the black inks used to compose characters in written texts have a surface-active agent content that lowers the surface tension of the medium to a value of between 40 and 45 mN/m, a value that allows limited “spreading” of the droplet, which thus results in a dot that has spread very little and has well-defined outlines.
For the sake of reference, it should be remembered that the surface tension τ of water is 71.9 mN/m at 25° C.
Colour inks, on the other hand, generally used to form pictures, have a surface-active agent content that brings the final surface tension to a value preferably between 30 and 35 mN/m, so that there is greater spreading on the paper and consequently the final image is homogeneous without presenting areas of lesser coverage. Spreading is still however influenced by other factors, such as for instance composition of the paper.
The surface-active agents used in inks for thermal ink jet printers are usually present in concentrations greater than or equal to their critical micellar concentration (cmc) in that system, to guarantee that the surface tension is maintained constant by the system above its cmc.
In fact, when there is a change in the number of molecules 74a of surface-active agent at the interface 61, the micelles 63 permit a restoral of the steady conditions by releasing some molecules 74b from the micelles at the interface, or by absorbing an excess of molecules in the micelles 63.
It is also known that the surface-active agent above its cmc is capable of minimizing the effect of one colour field encroaching upon another, as described for instance in U.S. Pat. No. 5,626,655.
During regular ejection of droplets 51 from a thermal type ink jet printing head, the generation and expulsion of consecutive droplets inside the nozzle 56 follow one another at a certain frequency, with the formation of a new air/ink interface each time a droplet separates.
The surface-active agent present in the ink may be capable of rearranging quickly, that is to say of migrating quickly to each new interface that is formed, or may rearrange slowly and not lower the surface tension value in good time. As the real situation while the head is operating is far removed from the equilibrium conditions of a static system, it is important to characterize the ink, not just with its static surface tension value τstat, but also with its dynamic value DST, as reported for instance in U.S. Pat. No. 4,492,968.
To guarantee a good wettability of the print medium by the ink, the surface-active agents used in the formulations for thermal ink jet printing often have a dynamic behaviour that guarantees fast rearrangement of the molecules of surface-active agent at the interface in such a way that the ejected droplet that impacts on the paper has a surface tension as close as possible to its static value, as reported in U.S. Pat. No. 5,650,543.
Fast rearrangement implies a DST value close to the τstat value. This low surface tension value in operating conditions in turn implies a low capillary pressure P, according to the formula A. The low capillary pressure P in turn implies low speed filling of the nozzle 56, and consequently a limited maximum ejection frequency.
In ink jet printing the current technology tends towards an ever higher working frequency (≧10 KHz) with respect to those of the previous technologies. High speed printing is becoming a characteristic qualifying the product; accordingly therefore a fundamental aspect of the technology is that of managing to eject droplets at high frequencies without detracting from the quality of the printed image.
The quality of the printed image also depends on regularity of the direction of the droplets ejected: if there are ink residues from previous ejections in and around the hole of the nozzle 56, this causes non-reproducible trajectories in the successive ejections, with resultant misalignment on the paper.
According to this invention, an ink is produced for thermal ink jet printing containing a surface-active agent with a “slow rearrangement” type dynamic behaviour.
Slow rearrangement implies a DST value greater than τstat. This high surface tension value under operating conditions in turn implies a high capillary pressure P, according to the formula A. The high capillary pressure P in turn implies high speed filling of the nozzle 56, and accordingly a high maximum ejection frequency.
Moreover, a high surface tension under dynamic head operating conditions allows the ejection hole to remain free of ink residues, as a result of the greater cohesion force between the molecules of the ink.